c++: Use error_at rather than warning_at for missing return in constexpr functions...

c++: Use error_at rather than warning_at for missing return in constexpr functions [PR96182]

For C++11 we already emit an error if a constexpr function doesn't contain
a return statement, because in C++11 that is the only thing it needs to
contain, but for C++14 we would normally issue a -Wreturn-type warning.

As mentioned by Jonathan, such constexpr functions are invalid, no
diagnostics required, because there doesn't exist any arguments for
which it would result in valid constant expression.

This raises it to an error in such cases.  The !LAMBDA_TYPE_P case
is to avoid error on g++.dg/pr81194.C where the user didn't write
constexpr anywhere and the operator() is compiler generated.

2020-07-31  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96182
	* decl.c (finish_function): In constexpr functions use for C++14 and
	later error instead of warning if no return statement is present and
	diagnose it regardless of warn_return_type.  Move the warn_return_type
	diagnostics earlier in the function.

	* g++.dg/cpp1y/constexpr-96182.C: New test.
	* g++.dg/other/error35.C (S<T>::g()): Add return statement.
	* g++.dg/cpp1y/pr63996.C (foo): Likewise.
	* g++.dg/cpp1y/constexpr-return2.C (f): Likewise.
	* g++.dg/cpp1y/var-templ44.C (make_array): Add throw 1.